Date of Award

3-2025

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD

Abstract

High-temperature tension-tension fatigue behavior of two advanced carbon/silicon carbide (C/SiC) ceramic matrix composites (CMCs) was investigated. Both composites were reinforced with T300 carbon fibers. Laminated carbon fiber preforms consisted of 24 plies of plain weave fabric in a 0/90 layup. The first composite, C/CVI-SiC, was processed by chemical vapor infiltration (CVI) of SiC into the fiber preform. Prior to the CVI process, the fiber preforms were coated with a duplex pyrolytic carbon and boron carbide fiber coating to create a weak fiber- matrix interphase. The second composite, C/HYPR-SiC, was also processed via CVI, but had an oxidation inhibited matrix comprising interchanging layers of silicon carbide (SiC) and boron carbide (B4C). Tensile stress-strain behavior of both composites was investigated, and the tensile properties measured at 1200°C. Tension-tension fatigue behavior was evaluated at 1200°C in air. Fatigue tests were conducted with a ratio of minimum to maximum stress of R = 0.1 at a frequency of 1 Hz. Fatigue stresses ranged from 49 to 200 MPa for the C/CVI-SiC and from 86 to 260 MPa for the C/HYPR-SiC. Fatigue runout was set to 200,000 cycles. All test specimens that achieved runout were subjected to tensile test at 1200℃ to evaluate the retained tensile strength. Fatigue performance of both composites was poor. For C/CVI-SiC the fatigue run-out was not achieved even at the low fatigue stress 49 MPa (14% UTS). For the C/HYPR-SiC the fatigue run-out was achieved only at 86 MPa (20% UTS). The C/HYPR-SiC composite retained 70-77% of its tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Post-test microstructural characterization revealed extreme fiber deterioration in the C/ CVI-SiC specimens, and less extreme, but still significant fiber deterioration in the C/HYPR-SiC specimens. Deterioration of the carbon fibers due to oxidation is behind the damage and failure of both composites at 1200°C in air.

AFIT Designator

AFIT-ENY-MS-25-M-129

DTIC Accession Number

AD1356367

Comments

An embargo was observed for posting this thesis.

Distribution A: Approved for Public Release, Distribution Unlimited.

PA clearance case 88ABW-2025-0270

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